Supply and demand.

SSD/NVMe prices have fallen dramatically and offer big performance, space and power efficiency benefits to most users. Many computer enclosures don’t even have bays to install 3.5" HDD’s anymore. If you’re building a PC for a user desktop or buying a laptop, most users will be more than satisfied with one or two NVMe sticks on the motherboard. There’s many more desktops and laptops than servers, so demand is higher and storage manufacturers have accommodated by shifting their production capacity to that product line.

So storage manufacturers have devoted more effort into maximizing materials and manufacturing efficiency/capacity to SSD/NVMe’s than HDD’s to accommodate consumer demand. HDD’s are now being considered more of a lower-volume niche/enterprise product, where capacity is more of a driver than price.